This invention relates to improved cathodes for use with electrolytic cells suited for the electrolysis of aqueous alkali metal salt solutions. More particularly, this invention relates to noble metal-coated electrodes having intermediate protective layers suitable for use as cathodes in electrolytic cells, enabling the cell to function more efficiently by reducing the cathode overvoltage.
The production of chlorine, hydrogen, chlorates, hydrochloric acids, and caustic solutions have been commercialized through the use of electrolytic processes. The electrolysis of alkali metal halide solutions has been undertaken for many years, with corresponding improvement in electrolytic cell design and manufacture. Mercury cells, diaphragm cells, membrane cells and specially designed cells have been employed extensively in the electrolysis of alkali metal halides. The electrolysis of alkali metal halides to produce alkali metal halates is accomplished in similar cells without diaphragms or membranes.
Each type of cell has advantages as well as disadvantages and each fits a specific industrial need, and such cells have been developed to a degree whereby high operating efficiencies have been obtained, and savings of energy, as well as increased useful life of the components of the electrolytic cells, have resulted. A common problem confronting the designers of the cells was the relatively limited life of the electrodes, especially anodes, due to their errosion or decomposition during cell operation. Consequently, great interest developed in anodes that would be free of the objectionable characteristics of the early graphite or carbon electrodes. Dimensionally stable anodes have been developed which have greatly overcome this problem. During the development of improved anodes for various electrolytic cells, minor attention had been given to the cathodes employed in the cells which traditionally have been composed of graphite, and later ferrous metals or titanium.
In an electrochemical cell, large quantities of electricity are used to conduct the reactions involved, and the savings of electricity, of whatever small amounts, is of great economic advantage to the operation of the cell. Therefore, the ability to effect savings in electricity in any step of the operation through cell operation, cell design or improvements in components is of utmost importance.
In terms of the actual voltages needed for the electrolytic reaction, the normal reversible potential for the reaction is increased by the values of the electrode potentials and ohmic drops. The increase in the value of the electrode potential over the normal reversible potential for the reaction is termed overvoltage. In other words, the difference between the electrode potential necessary for the flow of current and the equilibrium value of the electrode with no current flowing is the overvoltage of the electrode. Overvoltage is therefore related to such factors as the nature of the ion being discharged, the current density, the nature and surface structure of the electrode, the temperature, and the composition of the electrolyte. A great number of mechanisms have been proposed for the overvoltage-current density relationship at the electrodes. Overvoltage at the cathode in a chlor-alkali cell is due to the to the creation of the hydrogen atom and/or its subsequent formation into the hydrogen molecule. Cathode overvoltages can be reduced through the proper selection of materials, as it is well known that the hydrogen overvoltage is greatly dependent on the metal used for the electroactive surface.
Ideally a cathode should be constructed from materials that are inexpensive, easy to fabricate, mechanically strong and capable of withstanding the environmental conditions of an electrolytic cell. Iron or steel fulfills many of these requirements and has been the traditional material used since the advent of dimensionally stable anodes. When a chlor-alkali cell is bypassed or in an open circuit condition, the iron or steel cathodes become prone to electrolyte attack and their useful life is greatly reduced during this period. Metals more resistant to electrolyte attack than the iron or steel may be substituted, but usually are deficient in other characteristics. The overvoltage property of the metal is a major problem in these substitutions. Ferrous metal cathode have been used in commercial cells, but their overvoltage characteristics can be improved by replacing the iron with other metals, or by overcoating the iron with a high surface area electroactive material having lower overvoltage.
The use of noble metals has been investigated for cathode overvoltage reduction, and found to be quite beneficial, but due to the high cost of the metal, they have been avoided. U.S. Pat. No. 3,974,058, to Gokhale, issued Aug. 10, 1976, discloses a cathode for the electrolysis of alkali metal halide solutions comprising a metallic substrate, an intermediate layer of cobalt, and an overcoating of ruthenium. Similarly, Canadian Pat. No. 1,056,769, issued June 19, 1979, discloses that platinum-coated titanium cathodes have been employed in electrolytic cells, but have generally been found unsuitable due to excessive wear of the platinum coating. Other metals, such as cobalt and its alloys, and nickel and its alloys, reduce significantly the cathode overvoltage. These materials have been investigated and used, but the use of noble metals produces a more meaningful economic saving in the operation of chlor-alkali cells.
In a typical diaphragm type cell for the production of chlorine, the metal cathodes have been of a woven wire mesh construction. This woven wire mesh is most conveniently constructed from a ferrous metal. More recently, the cathodes have been manufactured in a foraminous form from a perforated and/or expanded metal sheet.
U.S. Pat. No. 3,859,196 by Ruthel et al., issued Jan. 7, 1975, is cited herein to show the state of the art. Attempts to use these ferrous metal configurations as a base material followed by overcoating with an improved surface having lower cathode overvoltage properties have been investigated. Improvements in current efficiency have been attained by the incorporation of noble metals into these cathode structures, but the techniques of manufacture, poor adherence of the coating to the substrate, and high material costs have negated their adoption. The present invention provides a procedure for the production of cathodes, suitable for use in electrolytic chlor-alkali or chlorate cells, that are economical to prepare, have good durability, and have reduced cathode overvoltages. Any of the foregoing ferrous metal configurations are suitable for purposes of this invention.